Release To Manufacturer
In computing, RTM means "release to manufacturer." It usually refers to a "final" version of software that the development company releases to the manufacturer for CD / DVD pressing. Can also loosely mean the "final" version of any particular piece of software, regardless of medium, and may also refer to hardware, depending on context.

Windows Vista has gone through many changes during its development phase, from Alpha all the way to RC2, and finally RTM. Vista RTM was released on November 8th, 2006.